Supported cobalt-containing hydrocarbon or Fischer-Tropsch synthesis (FTS) catalysts can be prepared by means of impregnation of a cobalt salt onto a catalyst support coupled with drying of the impregnated support, followed by calcination of the resultant dried impregnated support, to obtain a FTS catalyst precursor. The catalyst precursor is then activated under reduction conditions to obtain the FTS catalyst comprising cobalt metal crystallites dispersed on the support.
It is known that the activation conditions can be adapted to improve the activity of the eventual catalyst. The reduction conditions during activation are usually provided by hydrogen gas, but it is also known to use CO during activation. Khodakov A Y et al, in Journal of Catalysis, 277, 2011, 14-26, reported that standard H2-reduction of cobalt oxide [Co3O4(25% Co)/Pt(0.1%)/Al2O3] produces cobalt metal whereby, according to XRD measurements, the dominant phase is the cobalt face centred cubic (fcc) phase, accounting for approximately 80% of the cobalt metal with the remainder being the hexagonal close packed (hcp) phase. Treatment of the reduced (metallic) cobalt catalysts with pure CO at 220° C. leads to cobalt carbide (Co2C) formation which is inactive towards FTS. However, subsequent H2 treatment of the cobalt carbide at 220° C. results in the selective formation of the cobalt hcp phase. It is also accepted that the hcp phase is more active in FTS than the cobalt fcc phase and it is claimed by the authors that the H2 treatment of cobalt carbide followed by FTS showed 50% higher activity than the corresponding catalyst without CO-treatment.
It is clear from the Khodakov A Y et al teaching that the cobalt hcp phase is the desired phase for FTS since it affords higher FTS activity. The hydrogen treatment of cobalt carbide mainly produces cobalt in the hcp phase and it is known from the Khodakov A Y et al teaching that this conversion of the cobalt carbide to cobalt in the hcp phase occurs quickly at a temperature of 220° C.
The present inventors found that hydrogen treatment of cobalt carbide at a temperature above 300° C. also results in the cobalt hcp phase being formed in a similar manner to when the hydrogen treatment is performed below 250° C. No improvements were observed in the formation of the cobalt hcp phase at the higher temperatures, but it was most unexpectedly found that when this cobalt carbide was treated with hydrogen at a temperature above 300° C., the catalyst so formed had a higher FTS catalyst activity and/or lower methane selectivity. The reasons for these improvements are not clear at this stage. Furthermore, it was surprisingly found that the temperature at which the cobalt carbide forms also has an effect on the FTS catalyst activity and/or methane selectivity.
Catalyst activation procedures which involve CO treatment and hydrogen treatment are also described in WO 2006/087522; U.S. Pat. No. 6,509,382; WO 2011/027104; Oil and Gas Science and Technology—Rev, IFP, Vol. 64 (2009), No 1, pp. 49-62; and Catal. Today 164 (2011) 62. However, none of these documents discloses the processes of the present invention, and especially not the combination of conditions under which the carbide formation takes place and the cobalt carbide is subsequently treated with hydrogen.